In general, two processes are used for the introduction of sulfur into a phenate: a process in which an alkylphenol is firstly converted into its sulfide from using sulfur chloride or the like and the sulfide is then made into a metal salt; and another process in which sulfurization is carried out using elemental sulfur at the time of metal addition reaction or after metal salt formation. The present invention belongs to the latter sulfurization process. A number of methods for the production of an over-based phenate in which the latter sulfurization process was employed have been reported for a long time, such as a process by which an over-based phenate containing alkaline earth metals in two times or more higher amount than the theoretical amount can be produced with a single metal addition reaction. For example, Hori and Hayashida as disclosed in U.S. Pat. No. 4,123,371 have found that an unexpectedly highly basic alkaline earth metal phenate can be produced by carrying out the above reaction in the presence of excess phenols.
In addition, Hori and Ueda et al. as disclosed in U.S. Pat. No. 4,518,807 have found that conversion ratio of an alkaline earth metal reagent into final product per one reaction can be increased markedly without requiring the use of excess dihydric alcohol by the addition of an appropriate amount of water to a reaction system in which phenols, a dihydric alcohol and an alkaline earth metal reagent or an admixture thereof with sulfur are allowed to react with one another.
Contrary to the generally used processes in which an alkaline earth metal reagent is used in a large excess to an alkylphenol in order to obtain an over-based product, the just described two processes were successful in producing an over-based phenate by the use of a large excess of an alkylphenol against an alkaline earth metal reagent. In consequence, these two processes are advantageous in that kinds of reaction materials to be used can be limited to the minimum.
These processes, however, have disadvantages in that it is extremely difficult to produce an over-based sulfurized phenate having a higher basicity than those of the prior art products and, even if an over-based phenate is produced, the product cannot be handled easily because of its extremely high viscosity. Improvement of the basicity to a higher level than the prior art products and simultaneous decrease in the viscosity will have markedly large economic effects, because not only an over-based phenate can be handled easily but also its amount to be added to lubricating oil and therefore the amount of the expensive alkylphenol can be reduced.